A brain injury in a human may be caused by any number of events or conditions. In some cases, a brain injury may be caused by external mechanical force, such as rapid acceleration or deceleration, impact, blast waves, or penetration by a projectile. This type of acquired brain injury is generally known as traumatic brain injury. Another type of acquired brain injury involves biochemical forces, such as oxygen deprivation (hypoxia). Hypoxia generally refers to a deficiency in the amount of oxygen reaching body tissues or a condition of insufficient levels of oxygen in tissue or blood. Oxygen deprivation to the brain results in neuronal damage and death, which is in turn related to the extent of long term brain dysfunction. The concentration of certain biomarkers may become elevated as a result of neuronal damage and death. For example, tau proteins are associated with microtubules and localized in the axonal compartment of neurons. Tau is known to be elevated in the cerebrospinal fluid (CSF) of patients with neurodegenerative disease and head injuries. However, since such biomarkers must diffuse across the blood brain barrier, they may be present in the blood in proportion in extremely low concentrations that are not reliably measurable by typical conventional immunoassays. While the concentration of some biomarkers in the brain and central nervous system are known to increase with hypoxic events, the increased concentration has not been correlated with specific diagnostic indications and/or methods of treatment. In addition, while some methods exist for determining a brain injury in a patient and/or determining a course of treatment following a brain injury, many of the known methods are costly (e.g., magnetic resonance imaging) and/or provide unclear results and/or predictors. Accordingly, improved methods are needed.